1. Field of the Invention
The present invention relates to a slip suppression control system for a vehicle which is configured to control a wheel driving power according to a wheel traction state on a road surface.
2. Description of the Related Art
Conventionally, there is proposed a traction controller configured to reduce an engine driving power to restore a drive wheel traction on a road surface (e.g., see Japanese Laid-Open Patent Application Publication No. Hei 7-103009). This controller uses traction control for retarding an ignition timing of the engine with respect to an optimal timing to reduce the driving power if an increase rate of an engine speed exceeds a predetermined start threshold, thus inhibiting a slip. That is, when a predetermined start condition is met, the traction control is initiated by a background processing irrespective of a driver's operation.
However, preference is varied from driver to driver. Some drivers prefer to drive without reducing the driving power in a state where a drive wheel has slipped a little. To this end, a vehicle may be provided with an input switch for allowing a traction control function to be tuned ON/OFF and the traction control may not be initiated even if the start condition for the traction control is satisfied during driving when the driver turns OFF the input switch. Thus, the driver can enjoy driving according to the situation by turning ON the input switch to permit the traction control and by turning OFF the input switch to inhibit the traction control.
Since the OFF-operation of the input switch is a command for inhibiting the traction control, the traction control is forcibly terminated if the traction control is being executed at the time point when the OFF-operation occurs. That is, when the OFF-operation of the input switch is performed during driving of the vehicle, the traction control being executed may be forcibly terminated by the background processing and the driving power may be returned to one in a normal control state. In this case, the driving power is not suppressed by the forcible termination of the traction control even though a driver's throttle operation is fixed. As a result, the driving power is increased and a fluctuation occurs in the driving speed. This degrades the driving feel.